Currently, recovery and re-use of useful materials are investigated in products of various fields, and construction of system for sustainable material utilization is being requested. For the re-utilization of products of polymer materials, the products are re-used in their states (also including preparation of fiber materials from PET bottles) or recycled, from the standpoints of the effective utilization of finite carbon resources (C resources) and the saving of finite energy resources. The recycling method includes material recycling method, chemical recycling method and thermal recycling method, but these methods involve drawbacks such as a deterioration of quality such as a molecular weight loss in case of the material recycling method, a high energy consumption in case of the chemical recycling method, and a large carbon oxide gas generation in case of the thermal recycling method. For these reasons, a large proportion of the used plastics is in fact disposed of by combustion or in landfilling.
The above enzyme catalyst method can provide valuable intermediates of synthesis and materials for polymerization with a relatively low energy consumption, but applicable polymers are restricted because of substrate specificity of the enzyme.
Also from the viewpoint of effective utilization of carbon resources, it is ideal to finally return to the raw material by a chemical recycling method, in which known are a monomer recovery by a depolymerization reaction and a raw material monomer recovery by a chemical decomposition reaction. The chemical recycling (recycling to the raw material) of a polycondensation polymer generally requires an acid or base catalyst and a high temperature, and eventually a purifying operation. Also a neutralizing operation is generally necessary. For example a PET liquefying recycling, reported in a latest report, executes a conversion to an oily aromatic mixture by means of Ca(OH)2 catalyst and a high-temperature process at 700° C. (see, bellow identified Non-Patent Reference 1). However, the low-molecular compounds obtained in this way are unsuitable as a raw material for re-polymerization or synthesis.
Also in an example of polylactic acid, a technology of thermally decomposing poly-L-lactic acid to a lactide at a temperature of 280° C. or higher has been developed, it requires a high temperature and a detriment has been reported that the L-isomer of lactic acid is isomerized to a D,L-mixture. Also a method of processing a polylactic acid with ammonia water has been reported, but, isomerization takes place also in this case, and the generated lactic acid has to be neutralized and isolated from an aqueous solution, involving unnegligible energy. Various investigations have thus been made on the thermal decomposition of polylactic acid. However, various factors influence the thermal-decomposition and the behavior of the thermal decomposition is not uniform but still includes many unclarified phenomena. For example a clear matching cannot be recognized in a ceiling temperature, and very many mechanisms of thermal decomposition have been reported. It is also considered that plural reactions proceed simultaneously or in succession, and these facts have been an obstacle to a detailed dynamic analysis of polylactic acid (see, below identified Non-Patent References 2 and 3).
Considering the above, the present inventor has already proposed a polymer decomposition method and a polymer producing method of a complete recycling type, with a low energy consumption by the use of an enzyme. A decomposition method disclosed in the below identified Patent Reference 1 is a method of depolymerizing a trimethylene carbonate polymer in the presence of a hydrolyzing enzyme to produce trimethylene carbonate (1,3-dioxan-2-one), and the below identified Patent Reference 2 discloses a method of processing a caprolactone polymer with a hydrolyzing enzyme to producing dicaprolactone which is a cyclic dimer of caprolactone, and a method of polymerizing dicaprolactone in the presence of a hydrolyzing enzyme to produce a caprolactone polymer. Also the below identified Patent Reference 3 discloses a method of depolymerizing polyalkylene alkanoate or poly(3-hydroxyalkanoate) into an oligomer principally constituted of cyclic compounds, utilizing a hydrolyzing enzyme as described above and a method of polymerizing the cyclic oligomer.
The depolymerization in these methods is of a low energy consumption, because of the use of an enzyme, and the products obtained by the depolymerization can be again polymerized into polymers by an enzyme, so that these methods can effectively utilize the carbon resources without waste and can be considered as a polymer re-utilization of complete recycling type. Therefore, from the viewpoint of sustainable material utilization, the aforementioned methods are practical chemical recycling methods. Also the enzyme catalyst methods above are optimum for obtaining cyclic monomer or oligomer that is re-polymerizable.
On the other hand, as a detriment resulting from the substrate specificity of enzyme, these methods are limited to polymers susceptible to an enzyme reaction. Also the enzyme catalyst, being a protein catalyst, involves limitations inherent thereto, such as that the reaction temperature cannot be made very high and that it requires a long time to the completion of reaction for a polymer having a high crystallinity and a high intermolecular force.
Patent Reference 1: Japanese Patent Application Laid Open (JP-A) 2002-17384
Patent Reference 2: JP-A-2002-17385
Patent Reference 3: JP-A-2002-320499
Non-Patent Reference 1: T. Yoshioka et al., Chemistry Letters, Vol. 38, No. 3, p.282-283 (2004)
Non-Patent Reference 2: H. Nishida et al., Polymer Degradation and Stability, Vol. 70, p.485(2000)
Non-Patent Reference 3: H. Nishida et al., Polymer Degradation and Stability, Vol. 78, p.129 (2002)